1. Field of the Invention
The present invention relates to gate-controlled bidirectional semiconductor switching devices. More particularly, the invention relates to controlling the threshold of a current required to trigger such a device to conduct in one direction through the terminals with respect to the threshold of a current required to trigger the device to conduct in the other direction.
2. Discussion of the Prior Art
Bidirectional semiconductor switching devices are well known in the art. Such devices, having two main terminals and a third terminal, referred to as a gate terminal, are known as gate-controlled bidirectional switching devices. These devices find use, for example, in the bidirectional control of alternating currents.
The devices are typically formed from a thin wafer of semiconductor material of a negative conductivity type. Into this wafer selective base and then emitter regions of successively opposite conductivity types are formed adjacent both major surfaces of the wafer in accordance with prior art practices. The emitter regions lie partially laterally offset within the base regions whereby two laterally displaced parallel domains are formed, each leading vertically through the wafer from one major surface to the other. Each such domain includes four regions of successively opposite conductivity types starting with a region of positive conductivity type. In total, five regions of alternately opposite conductivity types are present across the thickness of the wafer, of which the emitter regions lie laterally offset within the base region.
Where it is desired that the current path in such a device extend the length of each of such domains through the thickness of the wafer, the two main terminals typically are located one on each of the major surfaces of the wafer. However, the geometry of the devices is not completely similar on each of the major surfaces of the wafer. The third terminal, the gate terminal, is typically located adjacent one of the major terminals on one major surface of the wafer. A signal in the form of a voltage resulting in a current flow into or out of the gate terminal triggers the device into a conductive state. The device then conducts a current in either one or the other direction through the two main terminals depending on the polarity of an applied bias voltage across the two main terminals at the time the device is triggered.
Because of the location of the gate terminal on one surface but not on the other, differences exist in the physical phenomena involved in triggering the device to conduct in one direction or in the other. When the device is triggered into conduction in the one direction, a blocking or a reverse biased junction borders a region of conductivity type which terminates partly in ohmic contact with the gate terminal. When the device is triggered into conduction in the other direction, a similar reverse biased junction is located remote from the gate, separated by a forward biased junction and the region of conductivity type located in the major portion and center of the body of the device.
It is therefore known that in such a device the trigger current required to initiate conduction in one direction tends to differ from that required to initiate conduction in the other direction. Attempts have already been made to equalize the threshold of the two trigger currents at which the device starts to conduct in the one as well as in the other direction.
A proposed prior art solution relates to arranging the gate electrode in proximity of the current paths for conduction in each of the directions through the device. However, for some modes of operation of prior art devices, it is nevertheless still customary to specify triggering characteristics in terms of a minimum trigger current or voltage at which the device becomes conductive in either direction. For circuit applications where low threshold or sensitive trigger currents are a requirement, a device with a trigger current of a sufficiently low threshold level in one direction may have a trigger current threshold level in the other direction which is so sensitive that the device may be triggered sporadically by noise pulses occurring in the respective circuit. While such noise problems may be overcome by additional buffer circuitry, it is nevertheless desirable to manufacture a gate-controlled bidirectional semiconductor switching device in a manner which provides substantially equal thresholds for the currents required to trigger the device into conduction in either of the two directions.